Ultrasonic transducers of this type are used in what are known as ball wire bonders. A wire bonder is a machine which is used to wire semiconductor chips after fitting thereof to a substrate. A ball wire bonder is a wire bonder in which the wire is guided through a longitudinal hole in the capillary and fastened to the connection points by means of the capillary, whereas in the case of what is known as a wedge wire bonder the capillary is replaced by a specific wedge tool which is also known as a sonotrode. The ultrasonic transducer comprises a horn and a piezoelectric drive for exciting an ultrasonic oscillation in the horn. The capillary is clamped to a tip of the horn. During the production of the wired connection between the connection point of the semiconductor chip and the connection point of the substrate, the end of the wire protruding from the capillary is initially melted to form a ball. Subsequently, the wire ball, which is referred to by specialists simply as the ball, is fastened to the connection point of the semiconductor chip by means of pressure and ultrasound. This involves crushing the wire ball. This process is referred to as ball-bonding. The wire is then drawn through to the required wire length, formed into a wire loop and welded to the connection point of the substrate. The latter part of the process is referred to as wedge-bonding. Once the wire has been fastened to the connection point of the substrate, the wire is torn off and the next bonding cycle can begin. In ball-bonding and in wedge-bonding, ultrasound is applied to the horn of the piezoelectric drive.
Ultrasonic transducers are known, for example from U.S. Pat. No. 5,603,445 U.S. Pat. No. 5,595,328, U.S. Pat. No. 5,364,005 U.S. Pat. No. 5,180,093, U.S. Pat. No. 5,368,216, U.S. Pat. No. 5,469,011, U.S. Pat. No. 5,578,888, U.S. Pat. No. 5,699,953 and U.S. Pat. No. 6,135,339.
The known ultrasonic transducers are designed in such a way that a continuous ultrasonic wave, the oscillations of which are directed in the longitudinal direction of the horn, forms in the horn. An ultrasonic wave of this type is also referred to as a longitudinal wave. The bonding head of a ball wire bonder allows the tip of the capillary to move in three spatial directions, whereby the bonding head has only three degrees of freedom. The horn ist therefore oriented, both in a conventional bonding head mounted on an XY table and in a rotative bonding head, such as are known from U.S. Pat. No. 5,114,302, U.S. Pat. No. 6,460,751 and WO 2006036669, more or less along a predetermined direction, whereas the wired connections can extend in all directions. This leads, on the one hand, to the adhesion of the wire to the connection points of the substrate not being of uniform strength in all wired connections and, on the other hand, to variations in the shape of the crushed ball.